Ultrasonic measurement is commonly used in industrial bulk material level measurements and flow control. The bulk material may be in granular, powder or liquid form and is stored in tanks and moved through pipes or conduits during various processes.
One of the most common technologies in ultrasonic measurement is “monopulse time of flight reflectometry” (“MTFR”) as disclosed in U.S. Pat. No. 4,890,266 to Woodward. Woodward discusses a system that produces a burst of ultrasound energy that is transmitted toward the surface of a material in a tank. The return echo is analyzed to provide information as to the distance to any reflecting targets. One drawback of the technique of Woodward is that the receiver required is so sensitive that it becomes saturated and insensitive to reflected energy immediately after transmission of the ultrasound burst. The result is that the receiver has a blind area extending 10 to 100 cm (or sometimes more depending on the particular device characteristics) along the axis of transmission. The sensor will not provide level measurement in this blind area. As a result of this limitation, the upper volume of the tanks is unavailable for use because the level of material in the tanks cannot be measured. Therefore, the practical storage volume of the tank is reduced by the inability of the receiver to measure the top of the tank. Another drawback is that since the time duration of the transmit pulse is short, and since the path losses for the return echo are usually large, the transmit pulse must be of high amplitude. A high amplitude pulse can only be generated by high power transmitter and amplifier which are relatively expensive.
Other methods are used in the prior art for measuring levels in tanks, including the use of radar frequency energy systems. The use of radar energy systems suffers from the same drawbacks as ultrasound systems. However, for radar frequency systems, an additional problem of timing arises. Since the speed of propagation for electromagnetic energy is much higher than for sound waves, it becomes difficult to capture reflected energy with sufficient precision to determine distances accurately. As a result, many radar frequency energy systems may not be used to determine the level in a tank due to the short distances involved. Another problem is that radar frequency systems are usually limited to conductive (i.e., metal) tanks. The reason is that leakage of radar energy from non-conductive tanks often exceeds allowable free space emissions for such energy. Still another problem with radar frequency energy systems is measuring levels of materials with low dielectric constants because they return only a very small echo. A small echo exacerbates the problems of requiring an initial high pulse energy and high system cost.
Another known ranging technique is “Time Domain Reflectometry” (TDR). In this technique a transmission line is placed in the tank and makes contact with the material to be measured. An electronic pulse is sent through the transmission line. The level of the material creates a slight impedance change in the transmission line thus reflecting a portion of the transmitted pulse. This technique suffers from problems in detecting materials with low dielectric constants. Many times materials with low dielectric constants do not reflect sufficient energy to be measurable, thus making the technique ineffective.
Other systems are known for measuring moving fluids in a pipe or conduit. These systems are known as Doppler flow meters. In a Doppler flow meter, a pulse of ultrasound energy is transmitted into a moving liquid medium. A portion of the pulse is reflected from suspended particles or bubbles in the moving fluid and picked up by a receiver. The receiver may be “range gated” in that it samples only a narrow time segment of the entire reflected energy, corresponding to a desired target range. The distance to and relative velocity of the suspended particles can be calculated from the Doppler shift of the reflected energy along with the range gate delay timing. By making a series of these measurements with different range gate timing delays, a profile of the liquid velocity may be generated. The profile, along with the depth of the liquid and the known cross sectional area of the flow conduit can be used to calculate the flow rate of the fluid. The flow rate is typically integrated over time to give the total volume of flow for some period of time. The major limitation of this type of system is that the range gate has a limited width in time, thus limiting the precision to which the returned frequency may be determined, and thus severely limiting the accuracy with which the flow may be calculated.
It is then a goal of the present invention to provide a direct sequence spread spectrum distance and velocity measurement system that increases dynamic range and reduces peak transmitted power.
It is another goal of the present invention to provide a direct sequence spread spectrum distance and velocity measurement system that minimizes the “blind area” and allows a more complete use of storage tank capacity.
It is another goal of the present invention to provide a direct sequence spread spectrum distance and velocity measurement system that will reduce radar energy escaping into the environment and can be applied in a non-conducting tank.
It is another goal of the present invention to provide a direct sequence spread spectrum distance and velocity measurement system that offers processing gain by reducing direct sequence spread spectrum side lobe interference through echo subtraction allowing detection and measurement of small echoes associated with low dielectric constant materials.
It is another goal of the present invention to provide a direct sequence spread spectrum distance and velocity measurement system that increases the precision of the Doppler based measurements by eliminating the limited time window of traditional systems by continuously examining the Doppler shifts at a given range from the sensor.